Liquid crystal display elements are widely used for displaying of personal computers and televisions, etc. The elements utilize the optical anisotropy, dielectric anisotropy and so on of liquid crystal coumpouns. Operating modes of LCD elements include the phase change (PC) mode, the twisted nematic (TN) mode, the super twisted nematic (STN) mode, the bistable twisted nematic (BTN) mode, the electrically controlled birefringence (ECB) mode, the optically compensated bend (OCB) mode, the in-plane switching (IPS) mode, the vertical alignment (VA) mode, and the polymer sustained alignment (PSA) mode, etc.
Such LCD element uses a liquid crystal composition having suitable physical properties. In order to improve the characteristics of the LCD element, the liquid crystal compounds contained in the composition preferably have the physical properties described in items 1) to 8):
1) high stability to heat and light, etc,
2) a high clearing point,
3) a low minimum temperature of liquid crystal phase,
4) a low viscosity (η),
5) a suitable optical anisotropy (Δn),
6) a large dielectric anisotropy (Δ∈),
7) a suitable elastic constant (K), and
8) good compatibility with other liquid crystal compounds.
Effects of the physical properties of the liquid crystal compound on the characteristics of the element are described below.
A compound having high stability to heat and light, etc. as described in 1) increases the voltage holding ratio of the element. Thus, the service life of the element becomes longer. A compound having a high clearing point as described in 2) broadens the temperature range in which the element can be used. A compound having a low minimum temperature of a liquid crystal phase such as a nematic phase or a smectic phase, especially a low minimum temperature of a nematic phase, as described in 3) also broadens the temperature range in which the element can be used. A compound having a low viscosity as described in 4) decreases the response time of the element.
A compound having a suitable optical anisotropy as described in 5) improves the contrast of the element. In accordance with the design of the element, a compound having a large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is required. When the response time is decreased by decreasing the cell gap of the element, a compound having a large optical anisotropy is suitable. A compound having a large dielectric anisotropy as described in 6) decreases the threshold voltage of the element. Thereby, the electric power consumption of the element is decreased.
With regard to 7), a compound having a large elastic constant decreases the response time of the element. A compound having a small elastic constant decreases the threshold voltage of the element. Therefore, a suitable elastic constant is required according to the characteristics to be improved. A compound having an good compatibility with other liquid crystal compounds as described in 8) is preferred because physical properties of the composition can be easily adjusted by mixing liquid crystal compounds having different physical properties.
A variety of liquid crystal compounds having large dielectric anisotropy have so far been prepared. Patent Document 1 and Non-Patent Document 1 have described a compound (S−1) having trifluoropropenyl. However, the compound does not have a sufficiently large dielectric anisotropy, so a liquid crystal composition containing the compound seems to be quite difficult to have a threshold voltage required by a commercially available element.

Patent Document 2 describes a compound (S-2) having pentafluoropropenyl. However, the compound does not have a sufficiently large dielectric anisotropy.
Moreover, Patent Document 3 also describes compounds (S-3) and (S-4) having pentafluoropropenyl or difluoroethyleneoxy. However, the compounds either do not have sufficiently large dielectric anisotropy, and additionally do not have sufficiently high stability to heat and light, etc. as described in 1).

Under such circumstance, development of a compound having good properties with regard to the above characteristics 1) to 8) is desired.